This invention relates to electric machines, and more particularly, to extending speed range operation of an electric machine using voltage mode operation.
It is known in the art relating to electric motors that polyphase permanent magnet (PM) brushless motors with a sinusoidal field offer the capability of providing low torque ripple, noise, and vibration in comparison with those with a trapezoidal field. Theoretically, if a motor controller can produce polyphase sinusoidal currents with the same frequency as that of the sinusoidal back-emfs, the torque output of the motor will be a constant, and zero torque ripple can be achieved. However, because of the practical limitations of motor design and controller implementation, there are always deviations from those assumptions of pure sinusoidal back-emf and current waveforms. The deviations will usually result in parasitic torque ripple components at various frequencies and magnitudes. The methods of torque control can influence the level of this parasitic torque ripple.
One method for torque control of a permanent magnet motor with a sinusoidal back-emf is accomplished by controlling the motor phase currents so that its current vector is aligned with the back-emf. This control method is known as the current mode control method. In such a method, the motor torque is proportional to the magnitude of the current. However, the current mode control method has some drawbacks. The current mode control method requires a complex controller for digital implementation. The controller requires two or more A/D channels to digitize the current feedback from current sensors. In a three phase system, it is convenient to transform the three-phase variables into a two dimensional d-q synchronous frame which is attached to the rotor and design the controller in the d-q frame. But, because of the considerable calculations and signal processing involved in performing the d-q transformation, reverse d-q transformation and P-I loop algorithms, a high speed processor such as a digital signal processor (DSP) has to be used to update the controller information every data sampling cycle.
One application for electric machine using voltage mode operation is the electric power steering (EPS), which has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow and is presently limited to small and midget-class cars because of the cost and performance challenges. Among the most challenging technical issues is the pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
The choice of motor type for an EPS is an important one because it determines the characteristics of the drive and the requirements on the power switching devices, controls, and consequently cost. Leading contenders are the Permanent Magnet (PM) brushless motor, the Permanent Magnet (PM) commutator-type and the switched reluctance (SR) motors, each of the three options has its own inherent advantages and limitations. The PM brushless motor, was chosen based on years of experimenting with commutator-type motors. The large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements. Additionally, the potential for brush breakage that may result in a rotor lock necessitates the use of a clutch to disconnect the motor from the drive shaft in case of brush failure. SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects. For column assist applications, the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brushless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also of future high performance EPS systems of medium and large vehicles.
Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brushless motors, they are no match to the smoothness and quietness of HPS with decades-long history of performance refinement efforts. Consumers are reluctant in compromising such features. Therefore, a new torque ripple free (TRF) system is needed which, as the name indicates, would eradicate the sources of torque ripple (under ideal conditions) and reduce the noise level considerably. The near-term goal is to enhance the performance of EPS systems with the long-term objective of increasing acceptability of EPS systems for broader usage.
Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used, namely:
1. Steering Feel: The key to the wider use of EPS is the ability to reproduce the smoothness feel of hydraulic steering systems at affordable prices. Pulsating torque produced by motors would be felt at the steering wheel, if not reduced to very low levels.
2. Audible Noise: The EPS audible noise emanates mainly from the motor and gearbox. The gear noise is usually mechanical and attributable to lash caused by manufacturing tolerances. The motor noise is mainly a result of structural vibration excited by torque pulsation and radial magnetic forces in brushless motors and by the commutator/brush assembly in commutator motors.
In order to better appreciate the elements of the new scheme, a more detailed discussion about the torque ripple and noise generation mechanisms with a focus on PM brushless motors is presented in the following sections.
Torque ripple Causes and Remedies
There are two sources for torque ripple in a conventional PM brushless motors, namely (1) cogging or detent torque, and (2) commutation torque.
The cogging torque is attributable to the magnetic interaction between the permanent magnets and the slotted structure of the armature. It exists in both brushless and brush-type machines at all speeds and loads, including no-load. The magnetic attraction force exerted on each individual stator tooth, as the magnet leading edge approaches, produces a positive torque, while the force between the tooth and the trailing edge causes a negative torque. The instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots. The amplitude of the cogging torque is affected by some design parameters, such as slot opening/slot pitch ratio; magnet strength; and air gap length, while its profile could be altered by varying the pole arc/pole pitch ratio. Careful selection of these parameters can lead to reduced cogging torque, but this approach is limited by practical and performance constraints.
A more common and effective approach is by skewing either the stator teeth or the rotor magnet longitudinally, which provides for a gradual transition as the magnet moves under a stator tooth. Theoretically, a skew amount of one slot pitch should eliminate cogging. However, because of practical factors such as magnetic leakage end effects, skew variation due to tolerances, and eccentricity, some undesirable cogging remains.
Typically, current control with phase advance is used in machines in order to extend the speed range of operation by controlling the angle xe2x80x9cxcex1xe2x80x9d between the current vector and back emf vector and through some degree of field weakening. Angle xcex1 is known as the xe2x80x9cpower anglexe2x80x9d because it affects the resultant electromechanical power. In the above case of phase advance the phase currents (current mode control) are required, and current sensors must be used. The benefit of phase advance, or power angle control (or current mode control), is in reducing inverter power rating (and hence, its cost) because of the lower phase current. The above method of controlling the machine currents based on torque and speed commands, as well as phase currents measurements, is known as the xe2x80x9ccurrent modexe2x80x9d of operation. In a xe2x80x9cvoltage modexe2x80x9d of operation, where the machine is controlled without phase current sensors, mainly for lower cost, phase advance has not been an option. Therefore, it is desirable to find a way to achieve similar results in controlling machine currents without using a current sensor.
A voltage mode control method and apparatus for extending speed range operation from a sinusoidally excited permanent magnet motor is disclosed. It has the advantage of achieving the required performance at a low cost because of the elimination of phase current sensors as well as reducing inverter switching devices ratings. The method includes a determination of a maximum value from a first set of parameters with each parameter having a known maximum value. As well as a reading of a second set of parameters is included. A computation of a first derived angle using the first set of parameters and the second set of parameters is then performed. A computation of a second derived angle using the first derived angle follows. A resultant output comprising a set of derived command voltages for controlling a power circuit is created whereby the power circuit can achieve lower required torque levels with lower current for power switches.